qemu-nbd: add doc for option -f

Signed-off-by: NWenchao Xia <xiawenc@linux.vnet.ibm.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

qemu-nbd.c

@@ -79,6 +79,7 @@ static void usage(const char *name)
 #endif
 "\n"
 "Block device options:\n"
+"  -f, --format=FORMAT  set image format (raw, qcow2, ...)\n"
 "  -r, --read-only      export read-only\n"
 "  -s, --snapshot       use FILE as an external snapshot, create a temporary\n"
 "                       file with backing_file=FILE, redirect the write to\n"

qemu-nbd.texi

@@ -22,6 +22,8 @@ Export QEMU disk image using NBD protocol.
   interface to bind to (default @samp{0.0.0.0})
 @item -k, --socket=@var{path}
   Use a unix socket with path @var{path}
+@item -f, --format=@var{format}
+  Set image format as @var{format}
 @item -r, --read-only
   export read-only
 @item -P, --partition=@var{num}